Cell tip growth underlies injury response of marine macroalgae

Regeneration is a widely observed phenomenon by which the integrity of an organism is recovered after damage. So far, studies on the molecular and cellular mechanisms of regeneration have been limited to a handful of model multicellular organisms. Here, we systematically surveyed the regeneration ability of marine macroalgae (Rhodophyta, Phaeophyceae, Chlorophyta) after thallus severing and applied live cell microscopy on them to uncover the cellular response to the damage. We observed three types of responses – budding, rhizoid formation and/or sporulation – in 25 species among 66 examined, demonstrating the high potential of regeneration of macroalgae. In contrast, callus formation, which often accompanies plant regeneration, was never observed. We monitored the cellular and nuclear dynamics during cell repair or rhizoid formation of four phylogenetically diverged Rhodophyta and Chlorophyta species (Colaconema sp., Dasya sessilis, Cladophora albida, Codium fragile). We observed tip growth of the cells near the damaged site as a common response, despite the difference in the number of nuclei and cells across species. Nuclear translocation follows tip growth, enabling overall uniform distribution of multinuclei (Dasya sessilis, Cladophora albida, Codium fragile) or central positioning of the mononucleus (Colaconema sp.). In contrast, the control of cell cycle events, such as nuclear division and septation, varied in these species. In Dasya sessilis, the division of multinuclei was synchronised, whereas it was not the case in Cladophora albida. Septation was tightly coupled with nuclear division in Colaconema and Dasya but not in others. These observations show that marine macroalgae utilise a variety of regeneration pathways, with some common features. This study also provides a novel methodology of live cell biology in macroalgae, offering a foundation for the future of this under-studied taxon.

Cytokinin Signaling Is Essential for Organ Formation in Marchantia polymorpha

Cytokinins are known to regulate various physiological events in plants. Cytokinin signaling is mediated by the phosphorelay system, one of the most ancient mechanisms controlling hormonal pathways in plants. The liverwort Marchantia polymorpha possesses all components necessary for cytokinin signaling; however, whether they respond to cytokinins and how the signaling is fine-tuned remain largely unknown. Here, we report cytokinin function in Marchantia development and organ formation. Our measurement of cytokinin species revealed that cis-zeatin is the most abundant cytokinin in Marchantia. We reduced the endogenous cytokinin level by overexpressing the gene for cytokinin oxidase, MpCKX, which inactivates cytokinins, and generated overexpression and knockout lines for type-A (MpRRA) and type-B (MpRRB) response regulators to manipulate the signaling. The overexpression lines of MpCKX and MpRRA, and the knockout lines of MpRRB, shared phenotypes such as inhibition of gemma cup formation, enhanced rhizoid formation and hyponastic thallus growth. Conversely, the knockout lines of MpRRA produced more gemma cups and exhibited epinastic thallus growth. MpRRA expression was elevated by cytokinin treatment and reduced by knocking out MpRRB, suggesting that MpRRA is upregulated by the MpRRB-mediated cytokinin signaling, which is antagonized by MpRRA. Our findings indicate that when plants moved onto land they already deployed the negative feedback loop of cytokinin signaling, which has an indispensable role in organogenesis.

Développement D’un Stock De Semences (Seedstocks) De L’algue Rouge Gelidium Corneum (Gelidiaceae, Rhodophyta)

Gelidium corneum is a species of red algae notable for its commercial important as an agarophyte in Morocco. Several regions from the Moroccan Atlantic show that this alga is an endangered species due to the excessive tearing. Hence, the repopulation of these areas is necessary. The in vitro culture of the species was carried out in three media: enriched seawater medium (PES medium (Provasoli Enriched Seawater, Provasoli 1968)), medium with seawater (SW) and medium with artificial seawater, with the addition of polyamines (putrescine (put), spermidine (spd), and spermine (spr)) as a growth regulator in the three media. The results obtained are very significant, especially in PES medium with a growth rate of 95%. Rhizoid formation and attachment of explants have been noted, especially in PES + Put medium.

The phaeophyte Hizikia fusiformis extracts suppress rhizoid and blade formation in seaweeds

<p class="MediumGrid2"><strong>Background</strong>: Plants, including marine algae, produce allelochemicals that influence the growth, survival, and reproduction of other organisms.</p><p class="MediumGrid2"><strong>Questions</strong>: To identify natural algicidal or antifouling allelochemicals, we screened 18 common seaweed extracts for suppression of rhizoid and blade production in a convenient <em>Porphyra suborbiculata</em> monospore assay.</p><p class="MediumGrid2"><strong>Species study and data description</strong>: Addition of extract from the most potent phaeophyte, <em>Hizikia fusiformis</em>, suppressed rhizoid formation, rhizoid number, rhizoid length, blade formation, and blade length.</p><p class="MediumGrid2"><strong>Study site and dates</strong>: Seaweed thalli for methanol extraction were collected on the coast of Korea from October 2012 to July 2015.</p><p class="MediumGrid2"><strong>Methods</strong>: Extracts were tested using the <em>P. suborbiculata</em> monospore assay system.</p><p class="MediumGrid2"><strong>Results</strong>: The 50 % suppression doses were 15 <em>µ</em>g ml^-1 for rhizoid formation, 2.4 <em>µ</em>g ml^-1 for rhizoid number, 13 <em>µ</em>g ml^-1 for rhizoid length, 6 <em>µ</em>g ml^-1 for blade formation, and 11 <em>µ</em>g ml^-1 for blade length. The <em>H. fusiformis</em> extract also suppressed rhizoid and blade production in leafy green (<em>Ulva pertusa</em>) and brown (<em>Undaria pinnatifida</em> and <em>Ecklonia cava</em>) seaweed spores, as well as suppressing diatom settlement.</p><p><strong>Conclusions</strong>: The allelochemicals that suppressed or eliminated competing seaweed species may be efficacious for new seaweed control technologies, including the development of antifouling or algicidal agents based on natural products.</p>

Cytoplasmic calcium gradients and calmodulin in the early development of the fucoid alga Pelvetia compressa

The predicted existence of cytoplasmic Ca2+ gradients during the photopolarization of the zygotes of the brown algae, Pelvetia and Fucus, has proved to be difficult to establish, and the downstream targets of the putative gradients are not known. We have used quantitative microinjection of the long excitation wavelength Ca2+ indicator, Calcium Crimson, and of antibodies against calmodulin to investigate these matters in the zygotes and early embryos of Pelvetia. We found that there is a window of cytoplasmic Calcium Crimson concentration that gives an adequate signal above autofluorescence yet allows normal development of the zygotes. As Calcium Crimson is not a ratiometric indicator, we injected other zygotes with a Ca2+-insensitive dye, rhodamine B, and imaged the cells at the same time that Calcium Crimson-injected cells were imaged. Ratios were calculated by dividing the averaged pixel values of Calcium Crimson images by the averaged pixel values of corresponding rhodamine B images. By this method, we observed the formation of a cytoplasmic Ca2+ gradient within one hour of the exposure of the cells to unilateral blue light during the photosensitive period. The region of high Ca2+ was localized to and predictive of the site of future rhizoid formation. We validated this somewhat indirect method by applying it to the growing rhizoid, where the existence of a tip-localized Ca2+ gradient is well established. The method clearly revealed the known gradient. The injection of ungerminated zygotes with antibodies made against Dictyostelium calmodulin inhibited germination, and this inhibition was abolished if the calmodulin antibodies were coinjected with an excess of purified maize calmodulin. Likewise, the growth of the rhizoids was inhibited by calmodulin antibody injections. The fungus-derived calmodulin antagonist, ophiobolin A, which has previously been shown to be a potent inhibitor of germination, also inhibited rhizoidal growth. Our results provide evidence that a cytoplasmic Ca2+ gradient is present during photopolarization and that calmodulin acts as a mediator of Ca2+ gradients throughout the early developmental processes of germination and rhizoidal growth in Pelvetia compressa.